A actuel developer version can you find in subversion. You can checkout all modules (tags, brach and trunk)
You can checkout with follow command:
svn co https://nagtrap.svn.sourceforge.net/svnroot/nagtrap nagtrap
After checkout you have a directory with the name nagtrap. Here can you find the actuel developer version in the directory trunk.
More information to subversion (SVN) can you find here.